Since the advent of low-cost laser diodes, their inherently low power densities have limited the usefulness of these devices. Various combinations of lenses and other optical devices have been used to combine multiple laser diode beams into fewer beams (e.g., a single beam) to increase power density. More commonly, multiple laser diodes or laser diode bar arrays simply serve as optical pumps for solid state lasers, rather than using their light directly in the laser machining or other high power laser process.
Using the laser diode light directly in laser machining and product marking could increase the efficiency and reduce the cost for such systems, but has proven to be a challenge due to the low average brightness or radiance of diode bar arrays. The low average radiance is due to the geometry of laser diode bars. Since a single emitting junction cannot provide the required high power, a stripe or series of long narrow emitters stacked end-to-end is usually fabricated into a high power laser diode bar. Each individual emitter is approximately 500 microns or more. Thus, although the radiance within each emitter aperture is quite high, the average radiance over the entire length of the stripe is low due to the dead space between the emitters.
A typical laser diode bar consists of a linear array of rectangular emitters, with each emitter having a narrow width about I micron and a length of several microns up to more than one hundred microns. Typically, the long dimension of the emitter is coplanar with the long dimension of the array, effectively producing a long thin line source as illustrated in FIG. 1. This is the easiest way to manufacture a high power laser diode bar in a single monolithic substrate. Good collimation of the light from the line source can be achieved in a narrow direction using a simple collimating lens, but not in the long direction. Due to the total length of the source in the long dimension, the collimated beam will have considerable divergence in this direction, as well as multiple dips in the intensity profile due to the non-emitting space between emitters.
In order to reduce this problem, some laser diode array manufacturers solder multiple emitter bars side-by-side in a stack as illustrated in FIG. 2. The amount of dead space is reduced relative to emitted power (although not completely eliminated) and the source size is more compact. Due to the soldering process, the emitter spacing is not nearly as uniform as that of FIG. 1, and thus is not conductive to coupling with microlens arrays for beam integration.